| Summary: | 碩士 === 國立東華大學 === 物理學系 === 107 === In this thesis, we report the investigation of the temperature-dependent magnetization (χ), specific heat (CP), electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) on the alkali-doped (Na and K) La0.80Sr0.20MnO3 manganite compounds in the temperature range of 10 K to 400 K. It was observed from these measurements that with increasing sodium content both the metal-insulator transition temperature (TMI) and the ferromagnetic-paramagnetic (TC) transition temperature shift towards to lower temperatures. It is found that the values of electrical resistivity increase with increasing Na content. This is attributed to the fact that the substitution of the larger Sr ions (1.18 Å) by smaller Na ions (1.02 Å) induces the distortion of MnO6 octahedra, leading to a weakening of double-exchange interaction. From specific heat measurements, the entropy change decreases slightly with increasing sodium content, indicating that the strength of magnetic ordering decreases with Na substitution, which is consistent with the magnetization measurements. The studied samples show a crossover around 200 K from a small positive value of Seebeck coefficient at low temperatures to a negative value at higher temperatures. This sign reversal is due to the change in the nature of charge carriers from holes to electrons. It is noted that the Seebeck coefficient of La0.80Na0.20MnO3 remain positive over the entire temperature range, this behavior is most likely due to an enhancement of the holes contribution with sodium substitution.
Analyses of electrical resistivity and Seebeck coefficient data confirm that the small polaron hopping (SPH) model is operative in the high-temperature insulating region, consistent with the positive slope in temperature-dependent thermal conductivity at high temperatures.
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